System and method for transmitting and receiving ultra wide band pulse or pulse sequence

ABSTRACT

The present invention provides a method for transmitting and receiving ultra wide band (UWB) signals. By this method, application systems such as wireless UWB communication, wireless UWB positioning and UWB target detecting radar can be achieved. In this method, an UWB device periodically generates pulse signals. In each pulse repetition period, an output signal can be a positive-polarity pulse (positive pulse), or a negative-polarity pulse (negative pulse), or an empty signal (empty signal) without any change. A method of UWB device controlling the output signal to shift between the positive pulse, negative pulse and empty signal is very simple and easy to implement. A transmitter can transmit any variable pulse sequence (the arranging order of the positive pulse, negative pulse and empty signal; the length of the sequence). One or more definite pulse sequences are used to constitute wireless information frame according to certain rules, and functions such as communications, positioning and target detection between one or more transmitters and one or more receivers can be realized. The prevent invention provides a method for generating UWB signals, as well as a low-cost and a high-performance UWB signal receiving method. According to the UWB signal generating and receiving method disclosed in the present invention, the system can very conveniently change (by software control) a central frequency and signal bandwidth of the UWB signals, and therefore can satisfy application requirements in different fields. A plurality of such transceivers can simultaneously work at the same waveband or different wavebands.

TECHNICAL FIELD

The present invention generally relates to wireless communication systems, wireless positioning systems, target detecting radar systems and the similar, such as an ultra wide band (UWB) system, comprising a mobile transceiver, a centralized transceiver and related apparatuses. Specifically, the present invention relates to technologies such as using UWB pulse sequence signals to transmit information between two UWB wireless transceivers, measuring a relative distance between two or more UWB wireless transceivers, using one or more UWB wireless transmitters to transmit signals and meanwhile using one or more UWB wireless receivers to receive signals for target detection.

BACKGROUND ART

Since radio technology appeared in the world in 1896, demands for radio communication apparatus have interacted with development of radio communication technologies. Performances of radio communication devices have been constantly improved, and meanwhile an unprecedentedly high requirement for radio communication technology has been raised. In 1948 Claud Shannon proposed a theoretical extremity [ref] of radio communication system performances, which is well-known Shannon Theorem or Shannon formula:

C=W*Log₂(1+S/N)   Formula 1: formula of Shannon Theorem

In the Formula 1, C denotes a theoretically achievable error-free extreme data transmission rate and is in an unit of bit per second (bps), W denotes a frequency band width occupied by the signals and is in an unit of Hetz (Hz). S/N denotes a signal to noise ration (SNR).

Shannon Theorem provides a theoretical extremity of radio communication system performances. Besides, if radio signals are used for positioning and target detection, the frequency band width of the radio signals is in inverse proportion to a time resolution of the system. Radio systems are generally approximately classified into narrow band systems, wide band systems and ultra wide band systems which definitions are respectively expressed in the following Formula 2, Formula 3 and Formula 4:

(f _(H) −f _(L))/f _(M)≦1%   Formula 2: Definition of narrow band radio systems

1%≦(f _(H) /f _(L))/f _(M)≦25%   Formula 3: Definition of wide band radio system

(f _(H) −f _(L))/f _(M)≦25% or (f _(H) −f _(L))≧500 MHz   Formula 4: definition of an ultra wide band radio system

The currently used radio systems are almost all narrow band and wide band systems. We hope to provide an UWB radio transceiver system based on pulses or pulse sequences, which can carry out operations such as radio data communication, radio positioning and radio target detection. Meanwhile, for different application demands and requirements of different regions for signal spectrum and control of transmission power, this system can dynamically configure parameters of the system such as signal central frequency, signal frequency band width and signal transmission power through a control interface of the system.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

A brief introduction of the accompanying drawings of the description is made first hereunder, and then embodiments of the present invention are described with reference to these accompanying drawings to illustrate principles and advantages of the present invention.

In the following drawings and description, the same reference number denotes the same device or a device with the similar function.

In the drawings,

FIG. 1 is a structure diagram showing an ultra wide band (UWB) transceiver system designed according to a preferred embodiment of the present invention;

The portion in the dotted line block of FIG. 2 is a structure diagram showing an UWB transmission radio frequency front end (0012) designed according to a preferred embodiment of the present invention;

The portion in the dotted line block of FIG. 3 is a structure diagram showing an UWB signal generator (0013) designed according to a preferred embodiment of the present invention;

The portion in the dotted line block of FIG. 4 is a block diagram showing an UWB signal controller (0014) designed according to a preferred embodiment of the present invention;

The portion in the dotted line block of FIG. 5 is a structure diagram showing an UWB receiver radio frequency front end (0022) designed according to a preferred embodiment of the present invention;

The portion in the dotted line block of FIG. 6 is a structure diagram showing an UWB correlation receiver (0023) designed according to a preferred embodiment of the present invention;

FIG. 7 shows time sequence and waveform of key signals of the UWB transmitter designed according to a preferred embodiment of the present invention;

FIG. 8 shows a time sequence and a waveform of key signals of the UWB receiver designed according to a preferred embodiment of the present invention;

FIG. 9 shows a correlation receiver circuit of the UWB receiver designed according to a preferred embodiment of the present invention;

FIG. 10 shows an UWB signal waveform and spectral distribution of a 1-2 GHz frequency band according to a preferred embodiment of the present invention, wherein a pulse width is about 14.2 ns and a signal width is about 320 MHz;

FIG. 11 shows an UWB signal waveform and spectral distribution of a 3-5 GHz frequency band according to a preferred embodiment of the present invention, wherein a pulse width is about 2.25 ns and a signal width is about 1.75 GHz;

FIG. 12 shows an UWB signal waveform and spectral distribution of a 6-10 GHz frequency band according to a preferred embodiment of the present invention, wherein a pulse width is about 1.125 ns and a signal width is about 3.5 GHz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention mainly relates to a method for generating an UWB signal and a method for receiving preprocessing. The UWB signal herein is a cyclically generated pulse signal. In each cycle, a positive pulse signal (positive pulse) is generated or a negative pulse signal (negative pulse) is generated or a neither positive nor negative pulse (empty signal) is generated. Therefore, the method for generating the UWB signals described hereunder is a method for generating a pulse sequence wherein the positive pulse, the negative pulse and the empty signal are arbitrarily arranged. The method for receiving preprocessing refers to processing the received pulse UWB signals. The present invention provides a cost-efficient correlation receiving method and a high-performance correlation receiving method. By these methods, the RF pulse UWB signal is converted into a digital baseband signal, and a subsequent baseband processor can carry out further processing to obtain information such as data, positioning and targets.

FIG. 1 is a structure diagram showing an ultra wide band (UWB) transceiver system designed according to a preferred embodiment of the present invention. The UWB transceiver consists of three basic parts: a transmitter (0010), a receiver (0020) and a signal processor (0030). The transmitter comprises a transmitting antenna (0011), a transmission radio frequency front end circuit (0012), a pulse signal generator (0013) and a pulse signal controller (0014). The receiver comprises a receiving antenna (0021), a receiving radio frequency front end circuit (0022), a correlation integral receiver (0023) and a reference signal generator (0024). The transmitting antenna and the receiving antenna can be two antennas or the same antenna wherein a transceiving switch decides whether the antenna is in a transmitting or receiving state. The signal processing circuit mainly comprises a transmission processor (0032), a transmission timing (0033), a receiving processor (0035) and a receiving timing (0036).

In regard to a radio system, a signal transmitting mode decides a basic structure of the system. Because a signal receiving mode varies in an extremely limited scope, signal transmitting modes are only given in physical layer (PHY) standards or protocols of the various current wireless antenna systems, and the signal receiving mode is decided by a designer of the system on his own, which is sufficient to ensure inter-communication and interconnection between apparatuses produced by different manufacturers. The signal transmitting mode of the present invention is first introduced hereunder, and then an optimized signal receiving mode is introduced.

To-be-transmitted information (0031) transmitted from an upper layer of the system takes information frame (data frame or timing frame) as an unit. The information frame can either be un-intermittently and continuously sent or sent intermittently at a regular time or at random. The data frame can be the transmitted data or can be considered as a timing (distance-measuring) mark; timing frame does not contain data or contains little data as timing (distance-measuring) mark. Like other wireless communications system, the information frame comprises a frame header and a frame body which are both a digital electrical level signal and converted into a transmission control signal after being processed by the transmission processor (0032) (encoding including error correction encoding or (and) spreading encoding or (and) modulation encoding). These transmission control signals must contain information about whether to transmit a pulse sequence and about a form of sending the pulse signal (positive pulse or negative pulse or empty signal) in the current pulse repetition period T upon transmitting the pulse sequence. The control signals are sent to the transmitter to control the transmission of the UWB pulse signals. The pulse signal controller (0014 in FIG. 4) generates 2*N+2 digital signals and corresponding status signal under control of the transmission control signals and the transmission timing signals sent from the transmission processor (0032), wherein 2*N+1 signals are used to control an output polarity of 2*N+1 weighted D/A converter. If a pulse needs to be transmitted, the 2*N+1 pulses respectively reverse the output polarity once at the offset locations (0, D, 2*D, . . . , N*D, , . . . 2*N*D) of the current time X*T (X is any positive integer and denotes a pulse number of the currently transmitted pulse sequence). The polarity of the output pulse is decided by the 2*N+2th signal. If the polarity of the pulse to be transmitted next is the same as the polarity of the currently transmitted pulse, this signal needs to reverse the output polarity once (X*T+(2*N+1)*D is the optimal time for reversing the output polarity) after the current pulse transmission ends and before next pulse transmission begins. If the currently transmitted pulse is an empty signal, all the signals needn't to be reversed. A minimum repetition period of the pulse signal is T_(min)=(2*N+2)*D. Generation of the above 2*N+2 signals requires the period to be greater than T_(min), and adjacent time delay is 2*N+2 multiple clock signals of D. Circuits for generating multiple clock signals can be a shift register (applicable for a low frequency system), a delay lock loop (applicable for a high frequency system) or other methods. Time delay D decides a central frequency (f_(M)=0.5/D) of the pulse signal: the greater D is, the lower the central frequency is; the less D is, the higher the central frequency is. N is a positive integer with a minimum value 1. N decides the bandwidth (f_(H)−f_(L)) of the pulse signal: the greater N is, the less the signal bandwidth is; the less N is, the greater the signal bandwidth is.

The pulse signal generator (0013 in FIG. 3) makes a summation from the output of the 2*N+1 weighted D/A converter under control of the digital signals sent from the pulse signal controller (0014 in FIG. 4) to generate a pulse sequence signal with alternating positive polarity and negative polarity. The positive-negative alternating pulse signals are converted by a polarity selection switch into any pulse sequence (positive pulse, negative pulse, null pulse). FIG. 7 shows time sequence and waveform of key signals of the transmitter designed according to a preferred embodiment of the present invention. In this preferred embodiment, N is 8. Signals (700, 701, . . . 708, . . . , 716) in FIG. 7 are respectively 2*8+1=17 output signals of the weighted D/A converter, and the signal (717) is a schematic view of pulse signals generated by weighting and summation.

Amplitudes of signals (700, 701, . . . , 708, . . . , 716) in FIG. 7 are decided by a selected pulse envelope function which is generally a bell shaped function. A central amplitude value of a normalized pulse envelope function is 1, the amplitude forwardly or backwardly from the center respectively attenuates smoothly from 1 to an enough small quantity &, the amplitude of the signals (700, 701, . . . , 708) smoothly increases from the small quantity & to 1. The amplitude of the signals (708, 709, . . . , 716) smoothly attenuates from 1 to the small quantity &. The pulse envelope function can either be a Gaussian function or other bell-shaped functions.

FIG. 10, FIG. 11 and FIG. 12 show a waveform and its spectrum of the pulse signal generated by using the Gaussian function as the envelope function. System parameters generating the signal as shown in FIG. 10 are N=19 and D=355 ps, a minimum pulse repetition period Tmin=14.2 ns, and signal bandwidth is approximately 320 MHz. System parameters generating the signal as shown in FIG. 11 are N=8 and D=125 ps, a minimum pulse repetition period Tmin=2.25 ns, and signal bandwidth is approximately 1.75 GHz. System parameters generating the signal as shown in FIG. 12 are N=8 and D=62.5 ps, a minimum pulse repetition period Tmin=1.125 ns, and signal bandwidth is approximately 3.5 GHz.

The transmission radio frequency front end circuit (0012 as shown in FIG. 2) carries out digital control and variable power amplification of the UWB pulse signal generated by the pulse signal generator (0013) (selection of a transmitting power) and then filters out-of-band noise via a bandpass filter (to reduce interference with other wireless systems). And finally the signal is transmitted out by the transmitting antenna (0011). The parameters N and D of the UWB transceiver according to the present invention are dynamically variable, that is, if needed, an application system can dynamically select a central frequency and frequency band width of the signal according to the needs. Therefore, the system might need a radio frequency electronic switch to select a bandpass filter and a transmitting antenna.

The UWB signal received by the receiving antenna (0021) is firstly sent to the receiving front end circuit for processing. The receiving antenna can be a passive antenna or active antenna. The active antenna itself carries a low noise amplifier (LNA) so that the receiver can operate at a lower signal to noise ratio (SNR). The signal received by the antenna needs to be filtered by the bandpass filter (0221) to remove the out-of-band interference, and then is subjected to further processing. Like the transmitter, the receiver might need to employ a radio frequency electronic switch to select different bandpass filters and receiving antennas. If the transmitter and the receiver share the bandpass filter and the antenna, a radio frequency electronic switch arrangement system needs to be employed to execute transmission or receiving.

The receiving radio frequency front-end circuit (0022) mainly comprises three parts: a bandpass filter (0221) and a filter selection switch (0222), a low noise amplifier (LNA) (0223) and a variable gain amplifier (VGA) (0224). LNA functions to reduce a noise coefficient of the system and improve the system performances. VGA functions to allow the amplitude of the input signal of the subsequent circuit always to approach maximum input signal amplitude as much as possible to maintain a dynamic range of the system.

The correlation integral receiver (0023) is a key component of the UWB wireless receiver. It is a well-known method that a four-quadrant analog multiplier carries out correlation computing of the received signal and the reference signal generated locally. The method of the present invention differs from other methods mainly at two aspects: 1) it uses two local reference signals I and Q as a set for correlation integral, wherein I and Q signals have the same waveform, but there is a time delay of D/2 between the two signals; 2) it uses a set of continuous waves or multiple sets of pulse waves as the local reference signal for correlation integral. One set of continuous wave reference signal is square wave or sinusoidal wave I and Q signals with a D*2 period. One set of pulse wave reference signal is pulse I and Q signals with a weighted time delay D and a repetition period T=D*(2*N+2). The time delay between two adjacent sets of pulse wave reference signals is D. The pulse wave I and Q signals can be signals with positive polarity pulse and negative polarity pulse alternated, or can be signals that all are positive polarity pulse signals or negative polarity pulse signals after being selected by a polarity selection switch.

FIG. 8 shows a time sequence and a waveform of key signals of the transmitter according to a preferred embodiment of the present invention. The signal (717) is a signal waveform transmitted by the transmitter. Signals (800) and (801) are a set of continuous wave I and Q signals (only square wave is drawn in the figure and sinusoidal wave is not drawn), signals (802) and (803) are a set of pulse wave I and Q signals (only positive polarity-negative polarity alternating pulse signal is drawn in the figure; pulse signals which all are positive polarity or negative polarity are not drawn in the figure). Each receiver requires pulse wave reference signal with a time delay D between adjacent sets of 2*N+2 sets. FIG. 9 is a correlation integrator circuit designed according to a preferred embodiment of the present invention.

A set of continuous wave reference signals is used for correlation integration. The receiver only needs two correlation integrators and two A/D converters, and what is output is a baseband signal modulated by binary phase shift keying (BPSK). A requirement for precision of the analog to digit converters (ADC) is approximately 6 bits. Therefore, a low-cost receiver can be realized, and a drawback thereof is that its multi-path detection capability is not as good as a receiver which employs multiple sets of pulse wave reference signals. Multiple sets of pulse wave reference signals are employed for correlation integration, wherein the receiver requires 2*(2*N+2) correlation integrators and 2*(2*N+2) ADCs, N≦11 for a f_(H)/f_(L)≈1.6 UWB receiver system, the number of the correlation integrators and ADCs is less than or equal to 48, and a requirement for precision of the ADC is approximately 3 bits. Such a receiver has a very powerful multi-path detection capability, and a system overhead is within an acceptable range. A high-performance communication receiver or a target detection receiver should choose to use multiple sets of pulse waves as reference signals for correlation integration.

If the continuous wave is selected as the reference signal, the reference signal generator (0024) can be composed of a voltage-controlled oscillator and a 90° phase shifter (a sinusoidal wave phase shifter) or D/2 time delayer (a square wave phase shifter, such as a time delay lock loop); if a pulse wave is selected as the reference signal, the reference signal generator (0024) can be the same circuit as the pulse signal generator (0013), wherein the reference signals are pulse signals which all are positive-polarity or negative-polarity or which are positive polarity-negative polarity alternating. The reference signal generator can be a pulse signal generator without a polarity selection switch and a spike elimination circuit, wherein the reference signals are positive polarity-negative polarity alternating pulse signals, and the system requires 2*(2*N+2) pulse signal generators.

The receiving processor (0035) can be a baseband processor of a binary phase shift keying (BPSK) system in any form. When multiple sets (or one set) of pulse wave reference signals are used, a special point is that thoughts should be given to requirements for realization of the baseband processor caused by selection of different reference signals. The pulse reference signals are selected from the following three kinds: 1) all-positive-polarity pulse reference signals, 2) all-negative-polarity pulse reference signals, and 3) positive polarity-negative polarity alternating pulse reference signals.

Reference Cited

C. E. SHANNON, “A Mathematical Theory of Communication”, Bell Syst. Techm. J., Vol 27, pp 379-423, July 1948, 623-656, October 1948. 

1. A method for generating an ultra wide band (UWB) radio frequency pulse and sequence, comprising: a) Providing a reference frequency; b) Generating an original polarity reversing signal according to the reference frequency and requirements about whether to transmit pulse; c) Generating multiple equal-delay polarity reversing signals from the original reversing signal; d) Carrying out adjacent opposite-symbol (positive, negative) weighting and summation of the multiple equal-delay polarity reversing signals, to generate positive-negative alternating UWB pulse (a weight needn't be changed when pulse is transmitted, and only the polarity of the polarity reversing signals needs to be changed); e) Selecting an output polarity of the UWB pulse according to a polarity requirement of the transmitted pulse to generate any UWB pulse sequence.
 2. The method according to claim 1, wherein a change rule of an absolute value of the amplitude of the multiple equal-delay polarity reversing signals is decided by a Gauss Function.
 3. The method according to claim 2, wherein the change rule of the absolute value of the amplitude of the multiple equal-delay polarity reversing signals is decided by another function similar to the Gauss Function (bell-shaped function).
 4. A method for controlling an UWB pulse central frequency and signal bandwidth, comprising: a) A process for operably generating multiple equal-delay polarity reversing signals with variable delay values (time delay is D; change of D can cause change of a signal central frequency); b) A process for operably generating multiple equal-delay polarity reversing signals with variable number of items (the number is 2*N+2; change of N can cause change of a signal frequency width); c) A process for operably performing weighting and summation of multiple equal-delay polarity reversing signals.
 5. A method for generating any pulse sequence, comprising: a) Generating positive-negative alternating UWB pulse signals based on the method claimed in claims 1, 2, 3 and 4; b) A process for operably changing the polarity of the UWB pulse signals (positive to negative, and negative to positive); c) A process for operably inserting a variable width time interval between adjacent pulses with opposite polarities (positive, negative); d) A process for operably converting a pulse sequence with adjacent hetero polarities (positive, negative, and null can be inserted therebetween) into any pulse sequence (positive, negative, null).
 6. An UWB radio frequency signal transmitting device, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a process of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null).
 7. An UWB radio frequency signal transmitting device, comprising: a) Means for providing a reference frequency; b) One or more sets of reference signal generating means; c) One or more sets of (low pass filter) means for bringing the reference signals into correlation operation and integration with the received radio frequency signals; d) Each set of reference signal is comprised of two signals with the same waveform but different delays (a delay difference D/2) or different phases (a phase difference 90 degrees).
 8. The method according to claim 7, wherein one set of continuous square waves are used as reference signals, and wherein the delay difference of two reference signals is one fourth signal period (the period is 2*D).
 9. The method according to claim 8, wherein one set of continuous sinusoidal waves are used as reference signals, and wherein a phase difference of two reference signals is π/2 (the period is 2*D).
 10. The method according to claim 7, wherein multiple sets (or one set) of positive-negative alternating pulse sequences are used as reference signals, the delay difference of the reference signals in the same set is D/2, the delay difference of reference signals in adjacent sets is D, a pulse repetition period of reference signals is D*(2*N+2), and 2*N+2 sets of reference signals are needed.
 11. The method according to claim 10, wherein multiple sets (or one set) of all-positive or all-negative pulse sequences are used as reference signals.
 12. An UWB radio frequency data communication device, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a process of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null) signals; and d) Means for operably using one set of continuous waves (square waves or sinusoidal waves) or multiple sets (or one set) of equal-delay pulse sequences (positive-negative alternating or all-positive or all-negative) to respectively conduct correlation operation and integration with the received UWB radio frequency signals.
 13. An UWB radio frequency distance-measuring and positioning device, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a process of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null) signals; and d) Means for operably using one set of continuous waves (square waves or sinusoidal waves) or multiple sets (or one set) of equal-delay pulse sequences (positive-negative alternating or all-positive or all-negative) to respectively conduct correlation operation and integration with the received UWB radio frequency signals.
 14. An UWB radio frequency communication system, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a process of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null) signals; and d) Means for operably using one set of continuous waves (square waves or sinusoidal waves) or multiple sets (or one set) of equal-delay pulse sequences (positive-negative alternating or all-positive or all-negative) to respectively conduct correlation operation and integration with the received UWB radio frequency signals.
 15. The system according to claim 14, wherein the system comprises more than one sub-systems recited in claim 14, and these sub-systems can simultaneously work in the same frequency band or different frequency bands.
 16. A single-frequency band UWB radio frequency distance-measuring and positioning system, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a method of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null) signals; and d) Means for operably using one set of continuous waves (square waves or sinusoidal waves) or multiple sets (or one set) of equal-delay pulse sequences (positive-negative alternating or all-positive or all-negative) to respectively conduct correlation operation and integration with the received UWB radio frequency signals.
 17. The system according to claim 16, wherein the system comprises more than one sub-systems recited in claim 16, and these sub-systems can simultaneously work in the same frequency band or different frequency bands.
 18. A single-frequency band UWB radio frequency target detecting system, comprising: a) Means for operably generating multiple equal-delay polarity reversing signals with variable number of items and variable delays; b) Means for generating UWB pulse signals based on a method of performing weighting and summation of multiple equal-delay polarity reversing signals; c) Means for operably converting positive-negative alternating pulse signals into any UWB pulse sequence (positive, negative, null) signals; and d) Means for operably using one set of continuous waves (square waves or sinusoidal waves) or multiple sets of equal-delay pulse sequences (positive-negative alternating or all-positive or all-negative) to respectively conduct correlation operation and integration with the received UWB radio frequency signals.
 19. The system according to claim 18, wherein the system comprises more than one sub-systems recited in claim 18, and these sub-systems can simultaneously work in the same frequency band or different frequency bands. 